The present invention relates to a viewing angle compensator for a liquid crystal display, and more particularly to a method for fabricating a viewing angle compensator for a liquid crystal display.
Because liquid crystal display has the advantages of small size, light weight, low driving voltage, low power consumption and good portability, it has been widely used as displays of a portable television, a laptop personal computer, a notebook, an electronic watch, a calculator, a mobile phone and an office automation device. The typical liquid crystal displays are generally classified into two types: a simple matrix type super twisted nematic liquid crystal display (STN-LCD) and an active matrix type thin film transistor-twist nematic liquid crystal display (TFT-TN LCD).
In a simple matrix type STN-LCD, the arrangement of the liquid crystal molecules is twist-aligned at an angle of about 180 to 240 degree in a direction from one substrate to the other. Such STN-LCD has advantages of great screen quality, high visual contrast and steep electro-optic characteristics. However, the STN-LCD also has some drawbacks such as a chromatic aberration and a narrow viewing angle. The phenomenon of chromatic aberration is caused when an incident light is at first linearly polarized upon transmitting through a polarizing plate and then enters through a liquid crystal cell to become an elliptically polarized light by the influence of non-uniform refraction and twist of the liquid crystal. Since the degree of elliptic polarization is dependent on the wavelength, the intensity of the transmitted light varies upon partially filtering portions of the wavelength through an analyzer arranged on the exit side of the liquid crystal cell, which results in a coloration of the display.
In an active matrix type of TFT-TN LCD, the arrangement of the liquid crystal molecules is twist-aligned at an angle of 90 degree in a direction from one substrate to the other. The TFT-TN LCD has a higher contrast and a wider viewing angle than those of the simple matrix type STN-LCD. The TFT-TN LCD also has advantages of high shading contrast (can be up to 100:1), rapid response and the characteristic of gray scale. However, the problem of the TFT-TN LCD involves that a reversal of brightness occurs at different viewing angles.
Please refer to FIG. 1, a relation between the voltage applied to the liquid crystal layer and the transmissivity at different viewing angles is illustrated. This relation is generally not linear. Such liquid crystal display comprises two substrates 1 in parallel to each other and liquid crystalline molecules 2. When the liquid crystal display is observed in the direction of A, the change of transmissivity can be easily distinguished between the state of power on and the state of power off. When the liquid crystal display is observed in the direction of B, the transmissivity is difficult to be reduced. With regard to the observation in the direction of C, the transmissivity reduces rapidly to zero as the driven voltage increases, however, some transmissivity appears when the voltage is further increased. It is apparent that the viewing angle is too narrow.
In connection with liquid crystal display, various display methods have been proposed to overcome the problem described above. For example, the compensators made of a uniaxial stretching polymer film, a liquid crystalline polymer, a polyimide film have been described. The uniaxial polymer film, such as polycarbonate, polyvinyl alcohol, polyethylene terephthalate and polysulfone, is a widely used material for manufacturing a compensator. The problems exist in point of manufacture and cost due to the difficulty of controlling the birefringence of the uniaxial stretching polymer film. The use of a liquid crystalline polymer involves problems; for example, the orienting process requires much time and labor and it is difficult to control the film thickness due to high viscosity thereof. However, a compensator made of a liquid crystalline polymer possesses better characteristics, e.g. birefringence property, than those of stretched films. The method for producing polyimide films is easy; however, the performance of improving viewing angle for a liquid crystal display is not satisfactory by using such polyimide films.
Therefore, the present invention provides an improved method for overcoming the problems described above.
An object of the present invention is to provide a method for producing a viewing angle compensator for liquid crystal display.
Another object of the present invention is to provide a compensator which is an aligned liquid crystal polymeric film for improving viewing angle and coloration for a liquid crystal display.
According to the present invention, the method for producing a viewing angle compensator includes steps of a) aligning two substrates, b) injecting a UV-curable liquid crystal monomer between the two substrates and c) polymerizing the UV-curable liquid crystal monomer to form a viewing angle compensator for the liquid crystal display.
In accordance with an aspect of the present invention, the step a) includes steps of a1) coating an alignment film on each of the two substrate and a2) orientatedly rubbing the alignment film.
Preferably, the alignment film is cross-linked with a polymeric material, which is one selected from a group consisting of ployimide and polyvinyl alcohol.
Preferably, each of the two substrates is a glass substrate.
More specially, each of the two substrates is a transparent plastic substrate, which is one selected from a group consisting of polycarbonate, polyethersulfone, polymethylmethacrylate and polytriacetyl cellulose.
In accordance with another aspect of the present invention, the two substrates are disposed in one of parallel alignment and twisted alignment.
In accordance with another aspect of the present invention, the twisted alignment of the liquid crystal monomer is twist-aligned at an angle of about 90 to 270 degree.
Preferably, the liquid crystal monomer is one selected from a group consisting of liquid crystal acrylate, liquid crystal methyl acrylate, liquid crystal ethylene oxide, liquid crystal bisacrylate, liquid crystal bismethylacrylate and liquid crystal bisethyleneoxide. The formula of such monomer is described as follows: 
where R is one of methylene and methylenoxy, X is a terminated radical selected from a group consisting of cyano, alkanoxy and alkyl, and Ar is a radical group selected from a group consisting of 
where A is one selected from a group consisting of hydrogen, alkyl and halogen.
In accordance with another aspect of the present invention, prior to the step b) further includes a step b1) mixing the UV-curable liquid crystal monomer and a non-liquid crystal monomer.
Preferably, the non-liquid crystal monomer is one selected from a group consisting of non-liquid crystal bisacrylate, non-liquid crystal bismethylacrylate and non-liquid crystal bisethyleneoxide. The formula of such monomer is described as follows: 
where R is one of methylene and methylenoxy.
In accordance with another aspect of the present invention, between the step b1) and b) further includes a step b2) adding a photoinitiator to the liquid crystal monomer.
Preferably, the photoinitiator is a radical photoinitiator selected from a group consisting of benzoin, benzil and benzophenone.
Preferably, the photoinitiator is a cationic photoinitiator selected from a group consisting of biphenyliodonium hexafluoroarsenate, diaryliodonium hexafluoroantimonate and triarylsulfonium hexafluoroantimonate.
In accordance with another aspect of the present invention, the step c) is conducted by polymerizing the UV-curable liquid crystal monomer at a polymerizing temperature.
Preferably, the polymerizing temperature is in the range between 30xc2x0 C. and 150xc2x0 C.
Another object of the present invention is to provide a compensator for a liquid crystal display, wherein the compensator is an aligned liquid crystal polymeric film constituted with a UV-curable liquid crystal monomer on a substrate.
In accordance with another aspect of the present invention, the aligned liquid crystal polymeric film of is disposed in one of parallel alignment and twisted alignment.
In accordance with another aspect of the present invention, the monomer is twist-aligned at an angle of 90 to 270 degree.
Preferably, the liquid crystal polymer of the compensator is one selected from a group consisting of liquid crystal acrylate, liquid crystal methyl acrylate, liquid crystal ethylene oxide, liquid crystal bisacrylate, liquid crystal bismethylacrylate and liquid crystal bisethylene oxide.
Preferably, the polymeric film of the compensator further includes a non-liquid crystal monomer, which is one selected from a group consisting of non-liquid crystal bisacrylate, non-liquid crystal bismethylacrylate and non-liquid crystal bisethyleneoxide.
In accordance with another aspect of the present invention, a retardation, defined as a product of a birefringence (xcex94n) and a film thickness (d) of the aligned liquid crystal polymeric film is in the range between 100 and 1000 nm.
In accordance with another aspect of the present invention, the compensator includes a transparent substrate.
Preferably, the transparent substrate is a glass substrate.
More specially, the transparent substrate is a plastic substrate selected from a group consisting of polycarbonate, polyethersulfone, polymethylmethacrylate and polytriacetyl cellulose.
The polymeric film obtained according to the method of the present invention shows highly optical anisotropy. The polymeric film having highly optical anisotropy can be used as a viewing angle compensator for STN-LN liquid crystal display. This compensator permits a black-and-white display and a wider viewing angle.
The polymeric film having highly optical anisotropy according to the present invention can be also used as a viewing angle compensator for TN liquid crystal display. This compensator permits a display having a wider viewing angle.
The above objects and advantages of the present invention will become more readily apparent to those ordinarily skilled in the art after reviewing the following detailed description and accompanying drawings, in which: